As for technologies for separation of materials which can be hardly separated by a generally used distillation process, it is known that various technologies such as adsorption, extraction, crystallization, membrane separation and the like can be used.
Para-xylene is a very important basic material in an industrial point. Separation of para-xylene used to be carried out by a crystallization process for some while and in recent years a SMB process, an energy-saving process has been being dominantly used. An adsorbent applicable to such SMB process should have excellent selective adsorption properties to a desired substance and be in the form of granules having high strength so as to be resistant to high temperature and high pressure. So far well-known adsorbent applicable to an SMB process for para-xylene separation is zeolite, which is mainly used in this field of art and particularly, a BaX type zeolite is most preferably used.
Zeolite is an crystalline aluminosillicate represented by the following formula 1:Mx/n[(AlO2)x(SiO2)y].wH2O  [Formula 1]
wherein, M is a cataion; n is an atomic valence; w is a molecular number of water of crystallization; x and y are integers varied depending on the crystalline structure.
Generally, zeolite has micropores having a diameter of 3-10 Å, having a molecular sieving effect, and owing to such structure, it further has a unique property of selectively adsorbing a certain molecules. Particularly, as a hydrophilic adsorbent, it shows strong adsorption to polar molecules such as water even under low partial pressure and high temperature conditions. In the meantime, although zeolite has a selective adsorption property as described above, its fine powdery form with the average particle size of several μm caused many inconveniences in practical use in adsorption or catalytic process.
Therefore, for the use of zeolite as a suitable adsorbent in practical industrial processes such as SMB process, it should undergo a molding process into a certain form with the addition of a binder.
As for so-far known conventional methods for the molding method of zeolite, for example, widely used is a method comprising the steps of: ion-exchanging zeolite utilizing a suitable cation and drying; adding a binder at the amount of about 30 wt % or less per particle for granulation and molding; subsequently calcinating the resulted product at 600-800° C.
As for the specific example of above-described conventional methods, a method for preparing an adsorbent by converting 4A type zeolite to 5A type zeolite via replacement of sodium ions in 4A type zeolite by calcium ions, then extruding and molding the converted 5A type zeolite using a natural clay of the kaolinite family as an inorganic binder, and calcinating the resultant. (see, U.S. Pat. Nos. 5,001,098 and 5,292,360, etc.). However, the above-mentioned method requires complicated filtering and washing process disadvantageously, since zeolite particles having several to several tens of μm size generated after the ion-exchange are present in a slurry phase. Moreover, the natural clay used as a binder for increasing the mechanical strength remains in the final product, thereby deteriorating the adsorption property.
In order to overcome such problems mentioned above, U.S. Pat. No. 6,743,745 discloses a method for preparing a zeolite granule in which the binder content can be minimized by blending approximately 2-15 wt % of highly dispersed attapulgite. From the above method, obtained is a zeolite granule in which about 10 wt % of the highly dispersed attapulgite remains inactive, and the remained highly dispersed attapulgite encompasses the zeolite component. Owing to such structures, it also has another disadvantage such that the diffusion of a gas component to be separated through the inside or outside of the granule is hindered, and thus the adsorption-desorption rate which is one of the important factors in SMB process is limited.
U.S. Pat. No. 4,603,040 discloses a method for a zeolite granule having improved adsorption property by forming a zeolite preform by using a kaolin binder, heating the zeolite preform at or above 600° C., placing the heated product in an aqueous alkaline solution for a long period of time such as 10 or more days so as to convert the binder component to zeolite. However, said method takes so long time to complete the process, it is deemed not to be industrially available in practical point of view.
For improving such method, for example, Korean Patent No. 10-0538961 discloses a method for preparing a low-content silica X type granular aggregates having inert binder at a low content, by blending it with about 15 wt % of natural clay (kaolin or montmorillonite type), calcinating the mixture at about 600° C., treating it with a mixed aqueous solution of sodium hydroxide and potassium hydroxide for at most 24 hours. However, said method also has disadvantages that, owing to the use of natural clay as a binder which causes shrinks during the calcinating process, the mechanical strength of the zeolite granule becomes decreased, thereby generating a great amount of dust, clogging most of the pores and thus making material transmission difficult.
In the meantime, DE patent No. 1,165,562 describes a method for preparing a binderless or binder-free zeolite by forming zeolite by using a silica sol as a binder and converting the silica component to zeolite by an aqueous solution of sodium aluminate. However, said method is reported to have defects such that the silica component served as a binder is exuded into the reaction mother liquor during said process and thus significantly decreases the strength of the final product.
As a method to overcome the problem of said DE patent, JP laid-open patent publication No. Heisei 6-53569 describes improvement in strength of a zeolite containing said silica binder by treating it with an alkali earth metal salt. According to said method, the zeolite component only inside the formed body can be converted to the zeolite in the form of alkali metal ion by ion exchange, and salts are penetrated and settled down in the voids formed between the silica component and the zeolite crystal particles, resulting in improvement in strength. However, material diffusion through the inside of the voids is inhibited, and thus the adsorption property is deteriorated.
Further, JP patent No. 4188050 describes a method for preparing a microspheric zeolite utilizing alumina sol. However, said method utilizes the alumina sol only as a binder and said alumina component remains as it is inside the resulted product, thereby lowering the adsorption property. Further, said method involves a complex process which comprises a first assembly process of a slurry mixture via a spray-drying process and then a second molding process for forming pellets.
As described above, the conventional adsorbents applied to a SMB process is prepared by blending a clay-type inorganic binder with a zeolite powder, molding and calcinating, or by treating a calcinated zeolite granules with an aqueous alkaline solution to convert a clay-type inorganic binder component into zeolite. However, these adsorbents prepared by such methods have problems such that impurities present in such clay-type inorganic binder still remain in the resulted granules and thus the improvement of adsorptive separation capability and mechanical strength is limited.